When bonding orthodontic appliances, a major tenet to be considered by the orthodontist is the location of the appliance in the mouth of the patient. In the case of bonded orthodontic brackets, this location is the position and orientation of each of the brackets on one of the teeth of the patient.
Most of the preadjusted appliances that are currently employed for maxillary application are designed to be located at the facial axes of the maxillary teeth of the patient. The facial axis, or FA point of a tooth illustrated as Point FA in FIG. 1A, is defined as dead center vertically on the clinical facial surface of a fully erupted crown and at the height of contour mesiodistally of the mid-developmental lobe of the tooth, for example, right maxillary central 20. This definition is based on the criteria that the plane of the archwire will pass through the point FA. Furthermore, appliances should be aligned angularly in the FA plane, which is the plane tangent to surface of the tooth at the FA point, at a line angle represented by line LA in FIG. 1A. This angle LA generally coincides with the line along the height of the contour of the tooth's mid-developmental lobe 21. So angularly aligned, the appliance will better deliver the appropriate final angular position or tip of the tooth. These criteria are referred to as the concept of morphological centering and angular alignment. This concept is quite often implemented visually by the orthodontist or other clinician who places the appliance on the tooth by making a visual determination of the location of the point FA and the orientation of the line LA. The visual implementation of this concept can be rendered difficult for the orthodontist by varying degrees of eruption that leaves an unerupted portion 22 of a tooth 20a, as illustrated in FIG. 1B, by virtue of chipped or worn incisal tooth edges 23 of a tooth 20b, as illustrated in FIG. 1C, or by virtue of the existence of other aberrations of the typical tooth profile. Teeth for mandibular application are also often visually placed in accordance with mandibular placement criteria.
While the concept of morphological centering and angular alignment is familiar to orthodontists, the clinical difficulty of achieving its placement goals is well known. Several approaches have been developed to alleviate this problem but all have drawbacks. The most common approach has been to use measuring instruments to position the appliances at fixed vertical heights. These heights typically represent something akin to the average distance from the incisal edge of a tooth to the FA point of the tooth, and are usually expressed in millimeters. The awkwardness of this approach is that teeth come in various sizes, which precludes placement of the appliance at the proportional center of the tooth in all but the truly average patient. Additionally, due to the highly probably presence of a malocclusion and to the lack of access because of the tooth's position in the mouth (e.g. as with posterior segments or crowding) there is often insufficient space to use these instruments effectively.
When the appliances are not placed at the design location, adverse effects occur with respect to the final positioning of the teeth. First, if the appliance such as an upper right central bracket 24 is placed incorrectly in the vertical plane, the faciolingual inclination of the tooth is effected, producing an inclination error 25 as illustrated in FIG. 1D. Secondly, at the same time, the apparent thickness of the appliance is effected, resulting in a labial-lingual offset error 26 from the desired placement of a tooth on the dental arch.
Another approach to this problem is that known as "indirect bonding." This approach involves positioning of the appliances on a model or cast of the patient and then using a transfer mechanism or tray to transfer the positioned appliances to corresponding positions on the teeth of the patient. The indirect bonding approach has its detracting features. For instance, often the tray does not seat fully, causing simultaneous incorrect placement of a multitude of individual appliances. Other problems include appliance adhesive failure and excessive "flash." Further, brackets are typically, although not always, placed by eye on the model, which offers little improvement over direct placement on most patients. These difficulties have been sufficient to severely limit the use of the indirect bonding technique.
An understanding of why the centering and aligning of appliances has been so clinically troublesome can be obtained by examining the morphology of the appliances and the dentition. Teeth, as most anatomical entities, have a generally flowing shape which does not lend itself to description or visualization using geometric determinants, as can be seen from the profile of a typical upper right cuspid 40 in FIG. 1E. Appliances such as brackets and their bonding pads, on the other hand, are typically generated from orthogonal geometric designs that lend themselves to ease of appliance manufacture, as can be seen from a typical upper right cuspid bracket 42. Further complicating this situation with respect to vertical placement is the varying torque or inclination angle of the archwire slot relative to the base of the appliance. This occurs whether an angle of a slot is cut in a bracket support, i.e. "torque in the face", or an angle is formed in the mounting surface of a bracket, e.g., "torque in the base". Because of this angle, the true plane of the archwire, which should intersect the FA point, is difficult for the orthodontist to visualize. Thus, positioning of the appliance, such as with the illustrated high torque upper right central bracket 24a, using either the bracket body or the facial view of the slot, will yield the positioning of the bracket on a tooth, such as tooth 20, with the archwire plane AWP intersecting the tooth 20 at a point displaced from the point FA, by an amount 28, as illustrated in FIG. 1F. Such a view afforded the clinician when placing the appliances often incorrectly influences the positioning. For the clinician to attempt to minimize this problem by viewing directly into the slot of the appliance during placement is at least awkward and not always possible clinically.
Additionally, when placing the appliance mesiodistally, the geometric appearance of the bracket and bonding pad can also be misleading, as seen when the rhomboid geometry of some individual appliances is compared to the dental anatomy. For example, FIG. 1G illustrates the difference between the shapes of orthogonal upper right cuspid bracket 42 and the anatomical shape of the upper right cuspid 40 with the bracket 42 correctly placed on mid-developmental lobe 41 that is not coincident with the mesiodistal center of the tooth. Further, when a clinician uses the bracket body portion of an appliance as the primary landmark, parallax is also a complicating circumstance that gives the clinician an incorrect apparent view of appliance position, as illustrated in FIG. 1H.
Notwithstanding the problems and disadvantages stated above, the concept of visual positioning of orthodontic appliances on the teeth of patients remains a technique that clinicians must use. Therefore, there remains a need for a solution to the problems of the prior art and for greater accuracy and reliability in visual appliance positioning.